AbstractThe effects of recombinant human hemopoietic growth factors on early and late human erythroid progenitors (BFU‐e and CFU‐e) were investigated in serum‐free cultures. Recombinant human erythropoietin (rhEpo) induced the formation of not only human CFU‐e‐derived colonies but also human BFU‐e‐derived bursts. Recombinant human interleukin 3 (rhIL‐3) alone did not induce the formation of human BFU‐e‐derived bursts and human CFU‐e‐derived colonies. In the presence of rhEpo, rhIL‐3 dose dependently increased the number of bursts stimulated by rhEpo, although rhIL‐3 did not have the augmentative effect on human CFU‐e growth. On the other hand, rhIL‐3 did not stimulate the formation of murine BFU‐e‐derived bursts, and murine IL‐3 did not stimulate the formation of human BFU‐e‐derived bursts. The results indicated that the burst‐promoting activity of IL‐3 was species‐specific between human and murine cells. Recombinant human GM‐CSF (rhGM‐CSF) or recombinant human G‐CSF (rhG‐CSF) failed to induce human burst formation and did not augment the effect of rhEpo on human burst formation. The results of the present study suggest that in vitro, IL‐3 can stimulate BFU‐e in collaboration with Epo, but GM‐CSF and G‐CSF